Matched microwave variable attenuator

Abstract

A variable attenuator includes a plurality of circuit cells, each containing a variable resistance shunt element, cascaded to form an artificial transmission line with distributed loss represented by the variable resistance shunt element. Parameters of the transmission line are chosen in a manner to establish a characteristic impedance that is substantially independent of the shunt resistance value and to provide low insertion reflection coefficients for all attenuation values.

I claim: 
     
       1. A variable attenuator comprising at least one circuit cell constructed and arranged to establish an artificial transmission line, each circuit cell of said at least one circuit cell having a variable shunt conductance element such that attenuation variable with settings of said variable shunt conductance is provided by power dissipation in said variable shunt conductance element in each of said at least one circuit cell. 
     
     
       2. An attenuator in accordance with claim 1 wherein each circuit cell of said at least one circuit cell is a ladder circuit having a series inductance and a shunt capacitance. 
     
     
       3. An attenuator in accordance with claim 2 wherein said variable shunt conductance element is an electrically variable resistor coupled in parallel with said shunt capacitance. 
     
     
       4. An attenuator in accordance with claim 3 wherein said electrically variable resistor is a FET. 
     
     
       5. An attenuator in accordance with claim 3 wherein said electrically variable resistor is a PIN diode. 
     
     
       6. An attenuator in accordance with claim 1 wherein each of said at least one circuit cell is a Pi circuit having first and second shunt capacitors and a series inductor coupled between said first and second shunt capacitors. 
     
     
       7. An attenuator in accordance with claim 6 wherein said variable shunt conductance element is an electrically variable resistor coupled in parallel with one of said first and second shunt capacitors. 
     
     
       8. An attenuator in accordance with claim 7 wherein said electrically variable resistor is a FET. 
     
     
       9. An attenuator in accordance with claim 7 wherein said electrically variable resistor is a PIN diode.